Tire non-uniformity relates to the symmetry (or lack of symmetry) relative to the tire's axis of revolution in certain quantifiable characteristics of a tire. Conventional tire building methods unfortunately have many opportunities for producing non-uniformities in tires. During revolution of the tires, non-uniformities present in the tire structure produce periodically-varying forces at the wheel axis. Tire non-uniformities are important when these force variations are transmitted as noticeable vibrations to the vehicle and vehicle occupants. These forces are transmitted through the suspension of the vehicle and may be felt in the seats and steering wheel of the vehicle or transmitted as noise in the passenger compartment. The amount of vibration transmitted to the vehicle occupants has been categorized as the “ride comfort” or “comfort” of the tires.
Tire uniformity parameters, or attributes, are generally categorized as dimensional or geometric variations (radial run out and lateral run out), mass variance, and rolling force variations (radial force variation, lateral force variation and tangential force variation, sometimes also called longitudinal or fore and aft force variation). Uniformity measurement machines often calculate the above and other uniformity characteristics by measuring force at a number of points around a tire as the tire is rotated about its axis to generate a uniformity waveform.
A tire uniformity waveform can be composed of a large number of harmonics, such as a first harmonic, second harmonic, third harmonic, etc. The individual harmonics can be obtained using Fourier decomposition of the uniformity waveform. The individual harmonics of a uniformity waveform are often expressed as uniformity vectors having a magnitude and phase angle. The magnitude can represent the maximum value of the harmonic. The phase angle can represent the location of the maximum value of the harmonic about the azimuth of the tire. Uniformity vectors can also be decomposed and expressed as rectangular x and y components.